Converting MGRS to Lat/Lon or UTM helped. I adapted the answer by Sasa Ivetic and the solution is somewhat obscure.
In order to convert coordinates from USNG/ MGRS to lon/lat, when there are far too many coordinates and you need to have it outomated:
1/ Get the GeographicLib package and use the GeoConvert utility.
2/ The general idea is to repeatedly call this utility from whatever tool you use (R, Python...), and supply user input and capture output.
3/ Specifically, and as an example, in Python 3.7, using the subprocess module, here are a couple of lines which can be looped:
from subprocess import Popen, PIPE
p = Popen('C:\\pkg-vc12-x64\\GeographicLib-1.49\\bin\\GeoConvert.exe', stdin=PIPE, stdout=PIPE)
p.communicate(b'48QYD307682')[0]
